In recent years, as to an electrophotographic image forming apparatus, an energy saving system has been demanded from the viewpoint of environmental consciousness with respect to the global environment. Improvement for a fixing system in an image forming apparatus, in which a great deal of energy is consumed, has been daily in progress, and low-temperature fixability with respect to toner has been highly demanded in order to realize energy saving in the fixing system. It is an effective means to realize low-temperature fixability that a polyester resin capable of changing a rapid fusing characteristic caused by raised temperature is utilized as a binder for the toner, but there may often appear a trouble such that it is difficult to obtain high-temperature offsetting resistance from a cause of an excellent fusing characteristic at high temperature; the resulting image has exhibited excessive gloss; and so forth.
In order to solve such a problem, proposed is a method of using a polyester resin having a crosslinking structure by which a large drop in elasticity at high temperature is inhibited (refer to Patent Document 1, for example).
However, a method of manufacturing toner disclosed in Patent Document 1 relates to a pulverization method, and in the case of the pulverization method, a great deal of energy is to be consumed in order to toner having a small particle diameter. Further, since downsizing of the toner particle diameter is limited, in recent years, often produced has been difficulty in high image-quality imaging as another attentive quality item which is desired for an electrophotographic image forming apparatus. On the other hand, in order to prepare toner having a small diameter, it is commonly known that it is effective to utilize a polymerization method in place of a pulverization method. The pulverization method is a collective term for a method of manufacturing toner via a chemical technique under a wet process condition (in water, an aqueous medium or an organic solvent). It appears that the toner is prepared with a polyester resin having a crosslinking structure via a polymerization method as a means of making low-temperature fixability and high-temperature offsetting resistance to be compatible, and of making inhibition of excessive gloss of the resulting image and achievement in high image-quality caused by downsizing of the toner particle diameter to be compatible, but in the case of a polymerization method, concerning inclusion of a step of finely dispersing a polyester resin, the polyester resin having a crosslinking structure can not be finely dispersed, or it is very difficult to make the foregoing performance items to be compatible at present since a large amount of energy is to be consumed.
In order to solve the above-described problem, for example, in Patent Documents 2 and 3, disclosed is a manufacturing method by which a crosslinking structure is formed during granulation of toner particles via a polymerization method by using a polyester resin into which an isocyanate group is introduced.
However, in the case of such a manufacturing method of toner, since the isocyanate group exhibits extremely high reactivity in its process, there appears a problem such that reaction is difficult to be controlled, whereby manufacturing can not be conducted stably.
Further, as a method of manufacturing toner containing a polyester resin having a crosslinking structure via a polymerization method, disclosed is a method of manufacturing toner having a core-shell structure in which a core particle composed of a crystalline polyester particle and a polyester resin particle containing at least a polymerizable unsaturated bond is prepared; a core-shell type coagulated particle is further prepared by attaching a polyester resin particle containing at least a polymerizable unsaturated bond onto the circumferential surface of the core particle; and the polymerizable unsaturated bond is subjected to radical polymerization via action of a radical polymerization initiator on this core-shell type coagulated particle to form a crosslinking structure.
This manufacturing method makes it possible to prepare the toner containing a polyester resin having a crosslinking structure via a polymerization method, and makes low-temperature fixability and high-temperature offsetting resistance to be compatible and makes inhibition of excessive gloss of the resulting image and achievement in high image-quality caused by downsizing of the toner particle diameter to be compatible, but on the other hand, depending on timing acting on radical polymerization initiator in a manufacturing method, there often appears a preparation trouble such that an aggregate of polyester resin particle-to-polyester resin particle each particle containing at least a polymerizable unsaturated bond is formed; the radical polymerization initiator is acted on this; a crosslinking body of this is formed; and so forth, resulting in lowering of manufacturing yield. Further, since a particle size distribution of prepared toner is broadened, often produced is a problem such that naturalness of density gradation in the resulting images originated by unevenness in electrification characteristic is deteriorated, and so forth.    Patent Document 1: Japanese Patent O.P.I. (Open to Public Inspection) Publication No. 2009-223281    Patent Document 2: Japanese Patent O.P.I. (Open to Public Inspection) Publication No. 2008-262166    Patent Document 3: Japanese Patent O.P.I. (Open to Public Inspection) Publication No. 2008-256913    Patent Document 4: Japanese Patent O.P.I. (Open to Public Inspection) Publication No. 2010-55092